Sounds like your default language is German. As Bob said, the language assignement is a character-level attribute, not a preference item, which lets you assigne different languages to "foreign" words in your text and ID will automatically use the correct dictionary.

To fix this for new docs, you need to change the language with no files open. You can find the language drop-down in the Control panel while in character mode, or inthe Character panel. You should also check your paragraph styles (especially Basic Paragraph).

To fix an existing file you can use Find/Change. Leave the find and change fields blank, but open the find format dialog and set the language to the German, then open the change format dialog and select English. Change all. If there are any words that SHOULD be German, you'll need to go back and fix them.

Jesse, I'm pretty sure this is a per document setting. At least that's what I experienced.

Means, if german is the system language, then ID will set the (nasty) 2006 Rechtschreibreform as default, but if you change it to Alte Rechtschreibung (the only real one) for the document you have currently open, then this is saved in the document and set again the next time you open it.